This invention relates to a flat-panel display device of a matrix type, and particularly to and a signal line driving circuit for driving signal lines of the flat-panel display device such as a liquid crystal display device.
For example, Jpn. Pat. Appln. KOKOKU Publication No. 7-40178 and Jpn. Pat. Appln. KOKOKU No. 5-5114 disclose a liquid crystal display device having a matrix type liquid crystal panel for displaying images for television broadcasting in a colored form. The liquid crystal panel has a matrix array of pixels, a plurality of scanning lines extending in a horizontal direction along the rows of the pixels, a plurality of signal lines extending in a vertical direction along the columns of the pixels, and a plurality of color filters covering the pixels. The pixels of each row have pixel electrodes which are charged by signal voltages applied to the respective signal lines while the pixels are being selected by a corresponding one of the scanning lines and the distribution of the light transmittance of the liquid crystal in the screen is controlled by the potentials of the pixel electrodes. The color filters are constructed by filters of three primary colors of red, green and blue arranged in regular color order in the horizontal direction and selectively transmit lights of specified wave lengths.
The liquid crystal display device has a signal line driver for driving the signal lines of the liquid crystal panel according to color video signals of red (R), green (G) and blue (B) derived by demodulating a video signal of television broadcasting form. The signal line driver has three bus lines for transmitting the RGB color video signals, and a plurality of sample-hold circuits which are connected to the three bus lines in regular order. The sample-hold circuits sequentially sample and hold RGB color video signals in one horizontal scanning period and output signal voltages to the signal lines after the whole sample-hold operation is completed.
The horizontal arrangement of the color filters does not necessarily mean that all of the pixels on the same column are covered with the filters of the same color. For example, the pixels on the first column are respectively covered with the color filters arranged in color order of RBRB - - - or BRGBRG - - - in the vertical direction. In the former case, the pixels on the first row are respectively covered with the color filters arranged in color order of RGBRGB - - - , the pixels on the second row are respectively covered with the color filters arranged in color order of BRGBRG - - - , and the pixels on the third and fourth rows, on the fifth and sixth rows, are covered with the color filters arranged in the same manner as described above.
Since the above positional relation lies between the pixels and the color filters, it becomes necessary to supply different color video signals to the respective signal lines as the row of the pixels to be selected by use of the scanning line is changed. To meet the above requirement, a switching unit comprising switches such as Field Effect Transistors (FETs) for switching the RGB color video signals supplied to the three bus lines is provided in the signal line driver.
In the above construction, the RGB color video signals are supplied to the sample-hold circuits via the bus lines from the switching unit. For example, if 100 sample-hold circuits are connected to each bus line, the bus line may have a stray capacitance of a relatively large value, for example, approx. 20 pF. The stray capacitance of the bus line and the output impedance of the switch constitute a low-pass filter. If the number of pixels is increased to enhance the resolution of the liquid crystal display device, it becomes necessary to reduce the output impedance of the switching unit so as to supply a signal of wide bandwidth to each sample-hold circuit.
To reduce the output impedance, for example, one may enlarge the gate width of a field effect transistor used for the switch. However, since this scheme increases the stray capacitance of the switch to be charged and discharged upon driving of the bus line, the power consumption is increased even under the same voltage and same frequency. Further, the bandwidth of a low-pass filter constructed by the stray capacitance of the switch and the output impedance of an RGB color video signal source is narrowed.
In order to prevent the bandwidth from being narrowed, it is necessary to further reduce the output impedance of the RGB color video signal source. For example, if the stray capacitance including the stray capacitance of the switching unit is increased by 1.5 times when an attempt is made to double the bandwidth by reducing the output impedance of the switching unit, it becomes necessary to lower the output impedance of the RGB color video signal source to one-third.
With the conventional signal line driver, since an increase in the number of pixels increases the stray capacitance which prevents the enlargement of the bandwidth or enhancement of the speed of the RGB color video signals, the power consumption is inevitably increased with an increase in the operation speed of the signal line driver.